1. Field of the Invention
The present invention relates to a process and to a device for measuring physical characteristics of a porous sample by centrifugal displacement of fluids.
2. Description of the Prior Art
It is important to determine the wettability of rocks with regard to the water and to the oil which may be contained therein. The rock must therefore be subjected to a drainage process, i.e. displacement of the fluids intended to decrease the water saturation, followed by an imbibition process, this term being to a displacement of the fluids which increases the water saturation (Sw) of the rock. The capillary pressure at one point is defined as the difference Pc at equilibrium between the pressure Po of the oil and the pressure Pw of the water. This parameter makes sense only if the two fluids are in the continuous phase in the porous medium. For a water wet medium, only positive values make sense. On the other hand, if the medium has a mixed wettability, the fluids can remain in the continuous phase for the positive and for the negative capillary pressures (Pc) as well.
For an application of this type, a prior art complete capillary pressure measuring cycle must therefore comprise (FIG. 1):
a) positive primary drainage of an initially 100% water-saturated sample (curve 11), PA1 b) positive imbibition (curve 2'), PA1 c) negative imbibition (curve 3'), PA1 d) negative drainage (curve 4'), and PA1 e) positive secondary drainage (curve 5'). PA1 a) installation of each sample previously saturated with conducting fluid and of the associated spacer in the first chamber of a vessel, this spacer being placed at a further distance from the hub of the centrifugation assembly than the sample, PA1 b) injection of conducting fluid in a known amount into the first chamber on the side thereof occupied by the spacer, PA1 c) increasing the centrifugation speed in successive stages until a capillary equilibrium is reached for each stage, and PA1 d) acquisition of the signals coming from the sonde, representative of the position variation of the interface between the two fluids, and determination of the volumes of fluid displaced by the measuring and control system. PA1 a) installing each sample and the associated spacer in the chamber of a vessel containing a conducting fluid, so that the spacer is closer to the hub than the sample, PA1 b) injecting, in each vessel, a known amount of the other fluid, PA1 c) increasing the centrifugation speed in successive stages until a capillary equilibrium is reached for each stage, and PA1 d) acquiring the signals from the sonde, representative of the position variations of the interface between the two fluids, and determining the volumes of fluid displaced by means of the measuring and control system.
Knowledge of various parameters and notably of the wettability of rocks is useful notably when enhanced recovery is to be performed in a formation, by draining the effluents contained therein by injecting a fluid under pressure, and when the most suitable fluid (water or gas) for effluents displacement is to be determined by means of preliminary tests.
The invention also finds applications in civil engineering for field hydrology purposes in order to evaluate the degree of pollution for example, or in the building industry to test construction materials, notably in order to select waterproofing treatments for example.
French Patent 2,763,690 filed by the assignee describes a method allowing measurement of physical characteristics of saturated rocks by subjecting them to a progressive-speed centrifugation and by measuring the amount of fluid displaced as a function of the rotating speed. The sample saturated with a liquid A for example is placed in an elongate container or vessel containing another fluid B of different density. The vessel is fastened to the end of a rotating arm and a centrifugal force is applied thereto so as to study fluid displacements in the sample during at least two distinct phases. During a first drainage phase, the assembly is then subjected to a centrifugal force applied along the length of the vessel so as to exert an expulsion force thereon, which tends to cause part of first fluid B to flow out. At the same time, some of fluid A flows into the sample. The two fluids move inside the sample until a position of equilibrium is reached, where the force due to the capillary pressure in the pores compensates for the centrifugal force exerted.
It is well-known that the capillary pressure P.sub.C at a distance R from the fulcrum pin, when it is positive, is expressed by the following relation: EQU P.sub.C (R)=1/2.DELTA..rho..omega.2(R.sup.2.sub.max -R.sup.2) P.sub.C (R.sub.max)=0
where .omega. is the angular rotating speed, R.sub.max is the distance from the base of the sample bar S to the fulcrum pin, .DELTA..rho. is the difference between the respective densities of the two fluids.
For negative values, the capillary pressure P.sub.C at a distance R from the fulcrum pin is: EQU P.sub.C (R)=1/2.DELTA..rho..omega..sup.2 (R.sup.2 min-R.sup.2) P.sub.C (R.sub.min)=0.
During the re-imbibition phase (curve 2'), the speed is decreased in order to study the re-integration of the initial fluid therein. With this type of method, local saturations are calculated by means of an inversion program from the total amount of water expelled from the sample.
The capillary pressure in the sample can be deduced from the precise measurement of the amount of initial fluid extracted as a function of the centrifugal force exerted and from the variation of the average fluid saturation S.sub.m of the sample as a function of the centrifugal force exerted, which is obtained by acoustic detection for example.
With a fluid-saturated sample, it can be seen (FIG. 1) that the saturation during the centrifugal drainage phase, for a determined radius r, decreases (curve 1') as the rotating speed w increases until a minimum value Si is reached. During this drainage phase, the rotating speed is increased in successive stages until a speed of 3500 rpm is reached for example. The fluid saturation variations are measured during the deceleration phase. A hysteresis phenomenon and a return, according to another variation curve (curve 2'), to a relative maximum value Sm are observed during the phase of re-imbibition of the porous material.
A system allowing maintenance of the drained fluid in contact with the sample bar is preferably used so that, when the deceleration phase starts, the bar can be properly re-imbibed. To ensure this maintenance, the system stabilizes the interface level between the two fluids at a minimum level where it is level with the base of the bar, i.e. at the furthest distance from the fulcrum pin (R.sub.max), at least throughout the deceleration phase. Such a system is easy to implement but it requires a pump borne by the arm and capable of withstanding the high accelerations required for implementation of the system, typically about 3000 g.
Patent application EN.97/06,434 filed by the assignee also describes a centrifugal type device allowing to measure physical characteristics of a porous solid sample by means of successive drainage and imbibition phases, in the presence of a first electricity-conducting fluid (such as brine for example) and of a second fluid of lower density than the first fluid (such as oil for example). It includes a rotatably mobile equipment comprising an elongate container or vessel (preferably at least a pair of vessels) provided with an inner cavity for the sample, each vessel being mounted swivelling at the end of an arm driven in rotation so as to apply a centrifugal force thereon in the direction of elongation of the vessel. A hydraulic system allows forcing displacement of the fluids and a capacitive sonde placed in the vessel, in the direction of elongation thereof, is used in order to continuously follow the displacements of the interface between the two fluids in the vessel. It is externally connected to a measuring device by an electro-hydraulic rotating connector.
This device can detect variations of the brine level with very high accuracy but its implementation requires a relatively costly rotating electro-hydraulic connector if perfect sealing is to be maintained.